R1:77ee518d88af

Make daemons ignore "Unreachable" commits and avoid duplicate work

Summary:
Ref T9028. This improves the daemon behavior for unreachable commits. There is still no way for commits to become marked unreachable on their own.

- When a daemon encounters an unreachable commit, fail permanently.
- When we revive a commit, queue new daemons to process it (since some of the daemons might have failed permanently the first time around).
- Before doing a step on a commit, check if the step has already been…